Party line identification system



June 4, 1946-. R. E. HERsEY 2,401,352

PARTY LINE IDENTIFICATION SYSTEM Filed sept. 7, 1944 l ATTORNEY Patented June 4, 1946 UNITED STATES PATENT 'OF FICE PARTY 'LINE IDENTIFICATION SYSTEM RalpliErHersey, Madison, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation 'of New York Application September 7, 1944, Serial No. 552,992

6 Claims. l1

This invention relates to telephone systems and more yparticularly to party line systems having testing means at the central'oflice for determining Vwhich of the telephones on the line is connected thereto for speech transmi-ssion, i. e., has its receiver off the hook. Y

A feature of the invention resides in providing (a) atfeach station on the line -a negative resistance element having a characteristic critical voltage individual thereto adapted to be'connected across the line in series with the associated station transmitter when thecorresponding receiver is removed from the switchhook; (l1) testing means at the central oliice for serially connecting a source of direct current, a marginal relay, and a resistor to the line, and for progressively reducing the effective resistance of said resistor until said relay operates; and (c) means,=respon sive to operation of said relay, for indicating the resistance reduction effected to cause operation of the relay, whereby the station whose receiver is off the hook is identified.

The invention will be understood from the following description when read in connection with the accompanying drawing which shows one embodiment thereof, i. e., a multiparty line L having four bridged stations PI, P2, P3 and P4 associated therewith each of which stations is provided with a negative resistance element TI, T2, etc., hereinafter referred to as thermistors and arranged'tofbe connected in series with the station transmitter when the receiver ofthe station is removed from the hook, and an operators cord circuit C provided with an identification key IK for associating an identication test circuit IT with the line to which the cord isconnected. vIt will be understood that the thermistors TI, T2, etc. each has a different critical voltage characteristic, i. e., requires a dilerent voltage to cause its normal high resistance to rapidly decrease to a relatively low resistance.

The operation of the system, as shown in the drawing, follows: Let it be assumed that the four thermistors TI, T2, etc. are so located on the line that TI, at station PI, requires the lowest voltage to cause its resistanceto change from a high toa low value, P2 requires a slightlyhigher potential, and so on to the last station P4 whose thermistor T4 requires a higher potential than any of the others preceding it.

Now, in case the receiver at station P2 is removed from its switchhook the line lamp I at the central olce will light n the usual manner to indicate a call. At this point it will also be `assumed that the thermistors TI, T2, etc. are so chosen that none of their critical voltages are higher than the line voltage of the central ollice battery B and consequently when any station calls its thermistorresistanceis'reducedto a suflicientlylow-voltage to permit satisfactory speech transmission. v

When Ythe line lamp `I is lighted the operator answers the call inthe usual manner byinserting the answering plug y2 in theline jack 3 and talks withvthe'calling subscriber. In case the connection vdesired by vthe'calling-subscriber is one requiring a charge Aand the operator desires to check the'identityof the calling station, she will operate identification key IK thereby disconnecting the portion Yof the cordecircuit to the left of the key and connecting the identification test crcuit IT to the calling line over the answering `plugfz whereupon the test battery TB is connected to the line in series with marginal relay 4 and test resistances TRI, TR2 and TRB. The voltage of test battery 'IBis sochosen that when connected to thelinein serieswth the testresistances TRI, TR2 and YTR3 `and relay 4, the :line voltage at any station will .be equal orgreater than the critical voltageof nthermistor .T I butless than the critical voltage of the others, and therefore thermistor TR2 at the calling station will immediately increase its resistance to a suiliciently high value to prevent suicient current from flowing through relay 4 to cause its operation. When key IK is operated it also closes its lowermost contacts 5 therebyclosing an operating circuitV for slow-operate relay 6 which includes the back contact 'l of. marginalrelay 4, which relay as beforenoted is in its non-operated condition. When relay 6 operates it locks over its inner upper Contact and contacts 5 'of 'key IK and closes its lowermost contact 8 thereby short-circuiting resistor TRI thereby increasing lthe line voltage applied to thermistor T2 at'the calling station to the critical voltage of that element whereupon its resistance falls to a low value thus permitting suicient current'to flow through `relay 4 to cause its operation.

Relay 4 in operating opens its contacts 1 and lcloses its contacts?! thereby completing a circuit to light station indicating lamplS2 which can be traced from ground, contacts 5 of 'key IK, contacts of relay 4,`contacts I of relay-B, contacts Il ofslow-operating relay I2, and lamp S2 to battery. The lighting of lamp S2 is an indication tothe operator .thatfthe receiver at .station P2 is olf Vthe lhook and she therefore Areleases key IK and Yproceeds to 4complete' the desired connection. The rrelease of key IK releases relays 4 and 6, Yextinguishes lamp S2 and restores the 3 line and cord circuit to the condition prior to the identication test.

If we assume that station P3 calls and the operator connects the identification test circuit IT to the line, relay I2 will operate and lock a short interval after relay 6 for the reason that relay 4 does not operate on the line voltage established when resistance TRI is short-circuited and relay 6 is operated and locked.

Relay I2 in operating closes its contacts I3 thereby short-circuiting resistance TR2 which further increases the line voltage'to the critical value of thermistor T3 which immediately lowers its resistance and causes relay 4 to'operate at the central oiiice. When relay 4 operates it closes its contacts 8 and completes a circuit to light lamp S3 which circuit includes contacts I0 of relay 6, contacts I 4 of relay I2, which is now operated, and contacts I6 of relay I5.

In a like manner if station P4 calls, relay 4 does not operate when relay l2 operates and therefore relay I soon operates in a circuit including contacts I9 of relay I2, thereby shortcircuiting the final test resistance TRS and causing the line voltage to increase to the critical value of thermistor TR4, thus causing relay 4 to operate to light lamp S4 in a circuit which includes contacts I0 of relay 6, contacts I4 of relay I2, and contacts Il of relay I5.

In case station TI calls, the initial voltage applied to the line in series with the total resistance of relay 4 and test resistance TRI, TR2 and TR3 is equal to or slightly greater than the voltage required to cause thermistor TRI to reduce its resistance to alow value and therefore relay lIl operates at once thus preventing 'relay E from operating. Relay 4 in closing its contacts 9 completes a circuit to light lamp SI which includes contacts I8 of relay 6.

It will be noted that other testing arrangements which involve the progressive step by step increase of a potential and its application to a line will cause an increase in the test current depending on the value of a, bridged resistance, and means for indicating the value of the test potential when this current increase occurs is within the scope of the present invention.

What is claimed is: d

1. In a telephone system in combination, a central office, a line terminating thereat having a plurality of telephones associated therewith each of which comprises a transmitter in series with a negative resistance element adapted to be connected in bridge of the line when the telephone is taken for use, each of said elements having a normal high resistance and a different critical point in its voltage resistance characteristic at which its resistance rapidly falls to a relatively low value, a source of potential, a resistance and a marginal relay at said central cnice, and means at the central oilice for 1testing said line to determine which of said telephones have been taken for use comprising a switching mechanism for serially connecting said source, resistance, and relay, with theline and for progressively reducing said resistance4 by vpredetermined s-tages until said relay operates, andmeans for indicating the stage Yto Which said resistance has been reduced when said relay operates.

2. In a telephone system, a central'oice, a line terminating thereat, a plurality of telephone stations thereon each of which is adapted to be connected thereto for conversation, and testing means controlled at the central ofiice for indicating which telephone is taken for use, said 4 means comprising a negative resistance element associated with each telephone in series with the respective transmitter, each element having a normal high resistance and a different critical point in its voltage-resistance characteristic at which point its resistance rapidly falls to a relatively low value, a source of direct current adapted to be connected to the line and progressively increased in voltage,'and a selective arrangement for indicating the line voltage at which the resistance of the element at the connected station falls to a low value.

3. In a telephone system, a central oice, a line terminating thereat, a plurality of telephone stations thereon each of which is adapted to be connected thereto for speech transmission and testing means controlled at the central ofce for identifying the station taken for use, said means comprising a negative resistance thermistor associated with each telephone in series with the respective transmitter, each thermistor having a normal high resistance and an individually different critical point in its voltage-resistance characteristic at which its resistance rapidly falls to a relatively low value, a source of variable test potential, means in said central oflice for applying said test potential to the line and for automatically increasing its voltage in successive stages, and identification means responsive to a decrease in the line resistance for discontinuing said voltage increase and for indicating the voltage stage at which said line resistance decrease occurs.

4. In a telephone system in combination, a central office, a line terminating thereat having a plurality of telephones associated therewith each of which comprises a transmitter in series with a negative resistance element adapted to be connected in bridge of the line when the telephone is taken for use, each of said elements having a normal high resistance and a different critical point in its voltage resistance characteristic at which its resistance rapidly falls to a relatively low value, a source of potential, a resistance and a marginal relay at said central oice and means at the central oice for testing said line todeterfmine which of said telephones has been taken for use comprising a switching mechanism for serially connecting said source, resistance, and relay, with the line, said source oi potential and associated resistance having such value that when one of said telephones is taken for use the potential across the respective negative resistance element is equal to the critical value of the element associated with the line having the lowest critical potential but below the critical value of the other, and said relay being so constructed that it will operate in circuit with any of said elements when its specified critical potentialis applied thereto, said switching mechanism being adapted, that when a telephone is taken for usc and said source of potential isconnected to the line, to progressively reduce said resistance in series with the source by predetermined stages as long as said relay remains unoperated, and means for selectively indicating the stage to whichsaid resistance has been reduced whenV said,` relay operates. A, I

5. In a telephone systemv in combinati'oma cen# tral oiice, a line terminating thereat having a plurality of telephones associated therewith each of which comprises a transmitter in series with a negative resistance element adapted to be connected in bridge of the line when the telephone is taken for use, each of said elements having a normal high resistance and a diiTerent critical point in its voltage resistance characteristic at which its resistance rapidly falls to a relatively 10W value, a source of potential, a resistance and a marginal relay at said central oiice, and means at the central oice for testing said line to determine which of said telephones has been taken for use comprising a switching mechanism for serially connecting said source, resistance and relay with the line, said source of potential and associated resistance having such value that when a telephone is taken for use the potential applied to the respective negative resistance element is equal to that required by the elements associated with the line having the lowest critical potential but lower than the critical potential of the other element, and said relay being so constructed that it will operate on current therethrough when a critical potential is applied to any one of said negative resistance elements, said switching mechanism being also adapted, upon the failure of said relay to operate to progressively reduce said resistance at the central office by predetermined failures until said relay operates, and means for selectively indicating the reduction in resistance eiective to cause operation of said relay.

6. In a party line identification system, a line, a plurality of stations thereon, a negative resistance element at each station adapted to be connected across the line in series with the associated transmitter when the corresponding receiver is removed from the sWitchhook, said elements having differing critical points in their voltage-resistance characteristic at which their resistance changes from a relatively high to a relatively low value, a central oilice at which said line terminates, a source of variable test potential, and means at the central oice for apply ing said test potential to the line and automatically increasing its voltage in successive stages until the resistance of the line decreases to a relatively low value, and other means responsive to a decrease in the line resistance for selectively indicating the voltage stage at which said line resistance decrease occurs.

RALPH E. HERSEY. 

